1. Field of the Invention
The present invention relates to an optical information recording/reproducing apparatus, and particularly to an objective lens actuator for an optical information recording/reproducing apparatus capable of optically writing and reading information on a disk-shape recording medium.
2. Description of the Related Art
An optical information recording/reproducing apparatus capable of optically writing and reading information on/from a disk-shape recording medium such as a magneto-optical disk records information, and reads information recorded, in a desired portion of a rotating recording medium by converging a light beam on the recording medium. When conducting the recording/reproducing of information, focusing errors may occur due to up-down motions of the warps of the disk-shape recording medium, and tracking errors due to eccentricity or the like of the disk-shape recording medium may occur. Such errors can be corrected by actuating an objective lens, which is used for converging a light beam, in a direction perpendicular to the surface of the recording medium (hereinafter referred to as "focusing direction F") and/or in a radial direction parallel to the surface of the recording medium (hereinafter referred to as "tracking direction T").
In recent years, there have been trends towards both the portable use and the personal use of an optical information recording/reproducing apparatus such as a miniature disk (MD) player or a data file device employing a magneto-optical disk. Accordingly, it is desirable to further reduce the size, in particular the thickness, of an objective lens actuator incorporated for actuating the objective lens in the focusing direction F and/or the tracking direction T.
FIGS. 23 and 24 show an exemplary configuration for a conventional objective lens actuator.
An objective lens 1 is supported by a lens holder 2. The optical axis of the objective lens 1 coincides with the focusing direction F. The objective lens 1 is disposed at a predetermined distance from the surface of a disk 15, in accordance with various factors such as the designs of the objective lens 1 and the focal length of the objective lens 1. A magnetic yoke 3b is fixed on a base 10. A magnet 4b is attached to a portion of the lens holder 2 where it faces the magnetic yoke 3b. A focusing coil 5b and a tracking coil 6b are wound around the magnetic yoke 3b, so that a magnetic circuit is constituted by the focusing coil 5b, the tracking coil 6b, the magnetic yoke 3b, and the magnet 4b. As a result of a driving force generated by this magnetic circuit, the lens holder 2 is moved in the focusing direction F and/or the tracking direction T. Moreover, the lens holder 2 is supported by elastic members 7a to 7d . The elastic members 7a to 7d are disposed substantially in parallel to one another. One end of each of elastic members 7a to 7d is attached to the lens holder 2 via a printed circuit board 8a or 8b. The other ends of the elastic members 7a to 7d are attached to a printed circuit board 8c via a stationary member 9. Thus, the lens holder 2 is supported by the elastic members 7a to 7b so as to be fixed on the base 10.
The base 10 of this objective lens actuator is attached to an optical unit tray 13. Provided on the optical unit tray 13 are a light source (not shown) for emitting a light beam 12, and a reflection mirror 11 for deflecting the light beam 12 so as to be incident to the objective lens 1, in addition to the above-described objective lens actuator. Also provided on the optical unit tray 13 are component elements such as a circuit for detecting error signals to be used for the focusing control and tracking control and for detecting the information recorded on the disk 15 by receiving a light beam reflected by the disk 15.
The disk 15 is accommodated in a cartridge 14. An opening 14a through which the light beam 12 is radiated on the disk 15 is opened in the cartridge 14. The opening 14a has a length W in a direction perpendicular to both the focusing direction F and the tracking direction T (i.e. a direction in which the light beam 12 enters the reflection mirror 11).
In order to reduce the thickness of the entire optical information recording/reproducing apparatus, it is preferable that the objective lens actuator fits within the opening 14a of the cartridge 14. The reason is that the objective lens actuator is thicker than any other component element provided on the optical unit tray 13, so that a total thickness H.sub.3 defined as a combination of the respective thicknesses of the objective lens actuator and the optical unit tray 13 (i.e. the distance from the lower face of the cartridge 14 to the lower face of the optical unit tray 13) is most critical to the thickness of the entire apparatus. However, there is a problem because the length W of the opening 14a is generally a small value, e.g. 17 mm in the case of an MD, so that it is difficult to fit the whole objective lens actuator within the opening 14a.
Accordingly, in a conventional optical information recording/reproducing apparatus, attempts are made to reduce the thicknesses of the objective lens actuator and the optical unit tray 13, for example, by configurating the lens holder 2 so as to project upward where it carries the objective lens 1, and by ensuring that the reflection mirror 11 can come inside a lower portion of the lens holder 2 under where the objective lens 1 is (i.e. so that the upper end of the reflection mirror 11 is located above the lowermost end of the lens holder 2), as shown in FIG. 23.
Correction of a focusing error or a tracking error is performed by moving the lens holder 2 carrying the objective lens 1 and the magnet 4b in the focusing direction F or the tracking direction T. Specifically, the actuation of the lens holder 2 along the focusing direction F is conducted by utilizing a driving force generated by an electrokinetic type transducer constituted by the focusing coil 5b, the magnetic yoke 3b, and the magnet 4b. On the other hand, the actuation of the lens holder 2 along the tracking direction T is conducted by utilizing a driving force generated by an electrokinetic type transducer constituted by the tracking coil 6b, the magnetic yoke 3b, and the magnet 4b. Since the lens holder 2 is supported by the four elastic members 7a to 7b disposed in parallel to one another, the lens holder 2 is always moved in parallel to the direction of the driving force.
However, the objective lens actuator of the configuration shown in FIG. 23 has a problem in that the stability of the servo controls, i.e. the focusing control and tracking control, may decrease. This is because the center of mass of a movable portion, which is composed essentially of the objective lens 1, the lens holder 2, the magnet 4b, and the printed circuit boards 8a and 8b, is at an offset with the driving force generated by the electrokinetic type transducer, possibly causing turbulence in the displacement-frequency characteristics. As shown in FIG. 25, one method to solve this problem is to dispose the electrokinetic type transducers on both sides of the lens holder 2 (along the direction in which the light beam 12 enters the reflection mirror 11) in such a manner that a middle point between the driving forces generated by the respective electrokinetic type transducers coincides with the center of mass of the movable portion, so that stable frequency characteristics can be obtained. However, according to the configuration shown in FIG. 25, it is impossible to allow the reflection mirror 11 to be inside the lower portion of the lens holder 2 (i.e. under the objective lens 1) in the same manner as shown in FIG. 23 because the reflection mirror 11, if situated inside the lower portion of the lens holder 2, would result in the light beam 12 being partially interrupted by the magnetic yoke 3a and the driving coils 5a and 6a. Therefore, the thickness of the objective lens actuator of the configuration shown in FIG. 25 cannot be reduced.
Moreover, in cases where an optical information recording/reproducing apparatus incorporating the above-mentioned objective lens actuator is employed as a portable device, there is a possibility for the disk to be accessed in a state where the tracking direction T is closer to or coincides with the direction of gravity. In such cases, the movable portion of the objective lens actuator may deviate from a neutral position due to the mass thereof, since as access operation is generally designed to be conducted when no tracking control is performed. Accordingly, in order to maintain the accuracy of the tracking control, it is required to design the movable portion of the objective lens actuator so as to be capable of moving in a broad area, in view of the above-mentioned possible deviation thereof, and also to design the diameter of the light beam 12 so as to be large. This will naturally hinder the reduction in thickness of the objective lens actuator.